I can certainly appreciate your frustration as you try to work through this rather complicated work, but I suspect you will recognize the answer to your question when you read it here and actually know more than you are giving yourself credit for knowing.

It is true that by the teacher's first definition of "icon" (famous people used as an example), you would naturally begin looking for proper nouns--particular people who have names and are famous. If you concentrate more on the second definition, "sinners," and couple it with the idea of sinners used as examples, I think you would be able to identify four specific groups of people who are both identified as sinners and used by Virgil and Dante as examples of behaviors which are not acceptable.

Dante's Inferno by Dante Alighieri is divided into "cantos," and Canto VII is set primarily in the fourth circle of Hell. The four kinds of sinners are found in pairs, and in both cases they are engaged in completely futile pursuits, an ironic and visual indication of their eternal punishment.

The first group who is acting out their eternal punishment includes the Avaricious and the Prodigal. The Avaricious are those whose lives were consumed with greed and acquisition; the Prodigal are those who lived their lives spending recklessly. These sins probably represent something more than just money; think of them as being either hoarders of the things they had to offer the world or squanderers who wasted their talents and gifts. They are, of course, connected because there is a kind of polarity to their sins. The hoarders have to pull heavy weights toward themselves (an appropriate move for hoarders) while the wastrels are forced to push heavy weights away from themselves with their chests. It is a painful and futile exercise, made worse by their constant yelling at each other.

Dante notices that some of them appear to be priests (are tonsured), and Virgil confirms that this is so. As usual in this work, the church is not shown in a positive light. Dante wishes he could identify some of the clerics specifically, but Virgil says:

the undiscerning life that made them filthy

now renders them unrecognizable.

The Avaricious and the Prodigal, and particularly the clerics among them, are literally filthy sinners.

When Dante and Virgil leave the fourth circle and head to the fifth, they have to cross the black waters of the river Styx. On their journey, Dante sees some people that he discovers are also sinners living out their eternal punishment. These people are wrestling around in the mud and the muck, but they are not obviously not wrestling for fun or sport. Instead, they are literally trying to kill one another as violently as possible. It is not surprising that Virgil identifies them as the Wrathful, obviously those who spent their lives full of contention and anger.

An amused Virgil adds that there is a second group kind of hiding (or more accurately, pouting) under the mud and the muck. This group, he says, are the Sullen, those who spent their lives in resentful silence, taking absolutely no pleasure in life. Their punishment is to recite hymns as they remain submerged in the mud; the result, obviously, is nothing but ineffectual glugs and gurgles.

All four of these icons (types of sinners) are being punished in ways that are specific to their sins, and Dante clearly intends for them to serve as examples to us all: Beware! The same fate awaits all who commit these sins.